Cadmium(II) Cysteine Complexes in the Solid State - ACS Publications

This is clear from the 113Cd NMR spectrum of the Cd(tu)2(NO3)2 complex (CdS2O4), where the peak maximum is at ∼280 ppm, while δiso = 123 ppm (Figur...
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Inorg. Chem. 2009, 48, 4219-4230

Cadmium(II) Cysteine Complexes in the Solid State: A Multispectroscopic Study Farideh Jalilehvand,*,† Vicky Mah,† Bonnie O. Leung,† Ja´nos Mink,‡,§ Guy M. Bernard,| and La´szlo´ Hajba§ Department of Chemistry, UniVersity of Calgary, Calgary, AB, Canada T2N 1N4, Department of Molecular Spectroscopy, Chemical Research Center of the Hungarian Academic of Sciences, P.O. Box 17, H-1525 Budapest, Hungary, Research Institute of Chemical and Process Engineering, Faculty of Information Technology, UniVersity of Pannonia, P.O. Box 158, H-8201 Veszpre´m, Hungary, and Department of Chemistry, UniVersity of Alberta, Edmonton, AB, Canada T6G 2G2 Received January 22, 2009

Cadmium(II) cysteinate compounds have recently been recognized to provide an environmentally friendly route for the production of CdS nanoparticles, used in semiconductors. In this article, we have studied the coordination for two cadmium(II) cysteinates, Cd(HCys)2 · H2O (1) and {Cd(HCys)2 · H2O}2 · H3O+ClO4- (2), by means of vibrational (Raman and IR absorption), solid-state NMR (113Cd and 13C), and Cd K- and L3-edge X-ray absorption spectroscopy. Indistinguishable Cd K-edge extended X-ray absorption fine structure (EXAFS) and Cd L3-edge X-ray absorption near edge structure (XANES) spectra were obtained for the two compounds, showing similar local structure around the cadmium(II) ions. The vibrational spectra show that the cysteine amine group is protonated (NH3+) and not involved in bonding. The 113Cd solid-state cross-polarization magic angle spinning NMR spectra showed a broad signal in the ∼500-700 ppm range, with the peak maximum at about 650 ppm, indicating three to four coordinated thiolate groups. Careful analyses of low-frequency Raman and far-IR spectra revealed bridging and terminal Cd-S vibrational bands. The average Cd-S distance of 2.52 ( 0.02 Å that constantly emerged from least-squares curve-fitting of the EXAFS spectra is consistent with CdS4 and CdS3O coordination. Both structural models yielded reasonable values for the refined parameters, with a slightly better fit for the CdS3O configuration, for which the Cd-O distance of 2.27 ( 0.04 Å was obtained. The Cd L3-edge XANES spectra of 1 and 2 resembled that of the CdS3O model compound and showed that the coordination around Cd(II) ions in 1 and 2 cannot be exclusively CdS4. The small separation of 176 cm-1 between the infrared symmetric and antisymmetric COO- stretching modes indicates monodentate or strongly asymmetrical bidentate coordination of a cysteine carboxylate group in the CdS3O units. The combined results are consistent with a “cyclic/cage” type of structure for both the amorphous solids 1 and 2, composed of CdS4 and CdS3O units with single thiolate (Cd-S-Cd) bridges, although a minor amount of cadmium(II) sites with CdS3O2-3 and CdS4O coordination geometries cannot be ruled out.

Introduction Sulfide and selenide semiconductors attract considerable interest due to their optical and electronic properties.1 A low * To whom correspondence should be addressed. Email: [email protected]. † University of Calgary. ‡ Chemical Research Center of the Hungarian Academic of Sciences. § University of Pannonia. | University of Alberta. (1) Wang, X.; Zhuang, J.; Peng, Q.; Li, Y. Langmuir 2006, 22, 7364– 7368.

10.1021/ic900145n CCC: $40.75 Published on Web 04/07/2009

 2009 American Chemical Society

cost and environmentally friendly route to prepare metal sulfide semiconductors could be via nanoparticles, synthesized by using small biomolecules such as cysteine as a source of sulfur.2-4 CdS nanoparticles, widely used in optical devices, can be prepared using cysteine and salts of cad(2) Zhang, B.; Ye, X.; Dai, W.; Hou, W.; Xie, Y. Chem.sEur. J. 2006, 12, 2337–2342. (3) Zhang, B.; Ye, X.; Hou, W.; Zhao, Y.; Xie, Y. J. Phys. Chem. B. 2006, 110, 8978–8985. (4) Chen, X.; Zhang, X.; Shi, C.; Li, X.; Qian, Y. Solid State Commun. 2005, 134, 613–615.

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Jalilehvand et al. Scheme 1. Previously Proposed Structures for Cd(HCys)2 (left) and [Cd{Cd(HCys)2}2][CdCl4] (right)7,9

mium(II) nitrate or chloride as precursors.5,6 Through a solvothermal process, Zhao and Huang obtained netted spherelike CdS nanostructures with sizes depending on the temperature (120-220 °C) as well as the Cd(II)/cysteine ratio and concluded that the CdS nanoparticles resulted from decomposition of the cadmium(II)-cysteine complexes at elevated temperatures.6 The structure of the cadmium(II) cysteine complexes is still an open question. Shindo and Brown obtained a 1:1 precipitate at pH ∼ 2 from equimolar amounts of CdCl2 and cysteine, with the empirical composition Cd(HCys)Cl. On the basis of vibrational IR spectroscopic data, they proposed mixed CdS4 and CdS2O2 coordination in a [Cd{Cd(HCys)2}2][CdCl4] structure (Scheme 1, right).7 The crystal structure of [Cd(HPen)Br(H2O)] · 2H2O (H2Pen ) penicillamine or 3,3′-dimethylcysteine) showed six-coordinated cadmium(II) ions surrounded by two bridging bromine atoms, a thiolate group from one penicillamine ligand, two oxygen atoms of the COO- group of another penicillamine molecule, and a water oxygen atom.8 Barrie et al. performed a solidstate 113Cd NMR study on a dehydrated Cd(HCys)2 compound, prepared by mixing cadmium(II) acetate and cysteine in the molar ratio 1:2, and proposed a polymeric structure with six-coordinated cadmium(II) ions, see Scheme 1 (left).9 In this work, we have developed a structural model for two solid cadmium(II)-cysteine compounds, Cd(HCys)2 · H2O (1) and {Cd(HCys)2 · H2O}2 · H3O+ClO4- (2), prepared from cadmium(II) acetate and perchlorate salts, respectively, based on results from a combination of spectroscopic methods. Experimental Section Sample Preparation. Cadmium(II) perchlorate hydrate, Cd(ClO4)2 · 6H2O, cadmium(II) acetate hydrate, Cd(CH3COO)2 · 2H2O, and L-cysteine (H2Cys, HO2CCH(NH2)CH2SH), were purchased from Sigma Aldrich and used without further purification. In order to prevent the oxidation of cysteine to cystine, samples were prepared under an argon atmosphere using water that had been boiled and purged with Ar to remove dissolved oxygen. The pH was monitored with a Corning Semi-Micro electrode. (5) Barglik-Chory, C.; Remenyi, C.; Strohm, H.; Mu¨ller, G. J. Phys. Chem. B. 2004, 108, 7637–7640. (6) Zhao, P.; Huang, K. Cryst. Growth Des. 2008, 8, 717–722. (7) Shindo, H.; Brown, T. L. J. Am. Chem. Soc. 1965, 87, 1904–1909. (8) Carty, A. J.; Taylor, N. J. Inorg. Chem. 1977, 16, 177–181. (9) Barrie, P. J.; Gyani, A.; Motevalli, M.; O’Brien, P. Inorg. Chem. 1993, 32, 3862–3867.

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Figure 1. Raman (left) and IR (right) spectra of solid (a) cysteine zwitterions, (b) {Cd(HCys)2 · H2O}2 · H3O+ClO4- (2), and (c) Cd(HCys)2 · H2O (1).

Cadmium(II) Cysteinate Solids. Cadmium(II) cysteinate, Cd(HCys)2 · H2O, was prepared as previously described:9 0.54 g (2.0 mmol) of cadmium(II) acetate hydrate was added to a solution of cysteine (0.48 g, 4.0 mmol) in 7 mL of oxygen-free water. A white precipitate immediately formed (pH ) 2.7), which was filtered, washed with water and ether, and dried in a vacuum. Anal. calcd for Cd(HCys)2 · 1.05H2O (CdC6H14.1N2O5.05S2): C, 19.40; H, 3.83; N, 7.54. Found: C, 19.24; H, 3.53; N, 7.00. DSC/TGA: 52-143 °C, 5.14% obsd for loss of water. A similar synthesis was performed using cadmium(II) perchlorate hydrate, and the precipitate formed (pH ) 1.6) was filtered, washed with water and ether, and dried in a vacuum. Anal. calcd for {Cd(HCys)2 · H2O}2 · H3O+ClO4- (Cd2C12H31N4O15S4Cl): C, 16.76; H, 3.16; N, 6.51. Found: C, 16.47; H, 3.22; N, 6.18. DSC/TGA: 52-187 °C, 4.86% obsd for loss of water. Raman and IR spectra of the solid confirmed the presence of perchlorate ions in the compound (Figure 1; Tables S-1 and S-2 and Figure S-1, Supporting Information). Attempts to crystallize these highly insoluble solids failed. Cadmium Reference Compounds. The crystalline compounds cadmium(II) perchlorate hydrate Cd(ClO4)2 · 6H2O (as CdO6 coordination model), hexakis(N,N-dimethylthioformamide) cadmium(II) perchlorate [Cd(SCHN(CH3)2)6](ClO4)2 (CdS6 model),10 tetraphenylphosphonium-tris(thioacetato) cadmate(II) [Ph4P][Cd(SCO(CH3))3] (CdS3O3 model),11 cadmium adamantane cage (Et3NH)4[S4Cd10(SPh)16] (CdS4 model),12 bis(thiosaccharinato)-bis(imidazole) cadmium(II) [Cd(tsac)2(Im)2] (CdS2N2 model),13 imidazolium tris(thiosaccharinato)aqua cadmate(II) (HIm)[Cd(tsac)3(H2O)] (CdS3O model),13 bis(nitrato)-bis(tetramethylthiourea) cadmium(II) [Cd(tmtu)2(NO3)2] (CdS2O4 model I),14 bis(nitrato)-bis(thiourea) cadmium(II) [Cd(tu)2(NO3)2] (CdS2O4 model II),15 bis(cysteaminate) cadmium(II) [Cd(SCH2CH2NH2)2] (CdS3N2 model),16 and bis(thiourea) cadmium(II) formate [Cd(tu)2(HCOO)2] (CdS4O2 model)17 were synthesized as previously described and verified by obtaining (10) Stålhandske, C. M. V.; Stålhandske, C. I.; Sandstro¨m, M.; Persson, I. Inorg. Chem. 1997, 36, 3167–3173. (11) Sampanthar, J. T.; Deivaraj, T. C.; Vittal, J. J.; Dean, P. A. W. J. Chem. Soc., Dalton Trans. 1999, 4419–4423. (12) Lee, G. S. H.; Fisher, K. J.; Vassallo, A. M.; Hanna, J. V.; Dance, I. G. Inorg. Chem. 1993, 32, 66–72. (13) Tarulli, S. H.; Quinzani, O. V.; Baran, E. J.; Piro, O. E.; Castellano, E. E. J. Mol. Struct. 2003, 656, 161–168. (14) Honkonen, R. S.; Marchetti, P. S.; Ellis, P. D. J. Am. Chem. Soc. 1986, 108, 912–915.

Cadmium(II) Cysteine Complexes the unit cell dimensions from X-ray crystallography for their known structures, Figure S-7 (Supporting Information). Vibrational Spectroscopy. The Raman spectra (range 100-3500 cm-1, resolution 4 cm-1, 1000 co-added scans) were recorded by means of a Bruker FRA 106 FT-Raman system equipped with a liquid-nitrogen-cooled Ge detector, exposing the solids contained in closed vials to the 1064 nm line (500 mW) from a YAG laser. The IR spectra (range 360-4000 cm-1, 4 cm-1 resolution, 1000 co-added scans) were measured using a Bruker Vertex70 FT-IR spectrometer equipped with a KBr beamsplitter and deuterotriglycine sulfate (DTGS) detector. Far-infrared spectra (50-700 cm-1) were recorded from polyethylene pellets with a Bio-Rad (Digilab) FTS-40 spectrometer equipped with an air-bearing interferometer, wire-mesh beamsplitter, high-pressure mercury source, and DTGS detector. Solid-State 113Cd and 13C NMR Spectroscopy. Solid-state cross-polarization magic angle spinning (CP MAS) experiments were carried out on a Bruker AMX300 spectrometer equipped with a BL4 MAS double-resonance broadband probe at resonance frequencies of 75.48 MHz for 13C and 66.59 MHz for 113Cd nuclei. The samples were packed into 4 mm ZrO rotors with MAS rates between 4 and 6 kHz (2 and 5 kHz for the [Cd(tu)2(HCOO)2] compound; tu ) thiourea). The Hartmann-Hahn condition for 13C experiments was optimized using solid glycine, which was also employed for secondary calibration by setting its carboxylate resonance to 176.2 ppm with respect to TMS.18 Spectra were acquired using a 4 µs 1H 90° pulse, 1 ms contact time, and 4 s recycle delay. For 113Cd experiments, solid Cd(ClO4)2 · 6H2O was used for calibration to 0 ppm and to set the Hartmann-Hahn condition.19 Spectra were acquired using a 4 µs 1H 90° pulse, 4 ms contact time, and 7 s recycle delay. For the [Cd(tu)2(NO3)2] and [Cd(tu)2(HCOO)2] solids, the principal components of the shielding tensors (δ11 g δ22 g δ33) were obtained by the method of Herzfeld and Berger,20 using the HBA 1.5 program.21 The output parameters were then used to simulate the spectra by means of the WSOLIDS 1 program.22 X-Ray Absorption Spectroscopy. Cadmium K-edge extended X-ray absorption fine structure (EXAFS) data were collected at beamline 2-3 at the Stanford Synchrotron Radiation Lightsource (SSRL) under dedicated conditions of 3.0 GeV and 70-100 mA. Higher harmonics were rejected by detuning the Si[220] doublecrystal monochromator to discard 50% of the maximum incident beam intensity. The spectra were recorded in transmission mode, with argon in the first ion chamber (I0), and krypton in the second (I1) and third (I2) ion chambers. Solid samples were ground finely, combined with BN, and placed in a 1 mm aluminum frame sealed with Mylar tape. Three to five scans were collected for each sample and averaged after external calibration of their energy scale by assigning the first inflection point of the Cd K-edge of a Cd foil to 26711.0 eV. (15) Petrova, R.; Bakardjieva, S.; Todorov, T. Z. Kristallogr. 2000, 215, 118. (16) Bharara, M. S.; Kim, C. H.; Parkin, S.; Atwood, D. A. Polyhedron 2005, 24, 865–871. (17) Nardelli, M.; Gasparri, G. F.; Boldrini, P. Acta Crystallogr. 1965, 18, 618–623. (18) Ye, C.; Fu, R.; Hu, J.; Hu, L.; Ding, S. Magn. Reson. Chem. 1993, 31, 699–704. (19) Mennitt, P. G.; Shatlock, M. P.; Bartsuka, V. J.; Maciel, G. E. J. Phys. Chem. 1981, 85, 2087–2091. (20) Herzfeld, J.; Berger, A. E. J. Chem. Phys. 1980, 73, 6021–6030. (21) Eichele, K.; Wasylishen, R. E. HBA, v. 1.5; University of Alberta: Edmonton, Canada; Universita¨t Tu¨bingen: Tu¨bingen, Germany, 2006. (22) Eichele, K.; Wasylishen, R. E. WSolids, v. 1.17.30; University of Alberta: Edmonton, Canada, 2001.

The Cd L3-edge X-ray absorption near edge structure (XANES) measurements were performed at beamline 9-A of the High Energy Accelerator Research Organization (Photon Factory), Tsukuba, Japan. The ring operates under dedicated conditions at 2.5 GeV and 350-400 mA. The data were collected in fluorescence mode with helium in the first ion chamber (I0) and an argon-filled Lytle detector (If). Higher harmonics through the Si[111] double crystal monochromator were rejected by means of nickel- and rhodiumcoated mirrors. The cadmium(II) cysteine samples and crystalline reference compounds were ground finely and dusted onto Mylar tape. For each sample, two to three scans were collected and averaged after external energy calibration relative to the first inflection point of the Cd L3-edge of a Cd foil at 3537.6 eV. XAS Data Analysis. The data treatment was performed using WinXAS 3.1.23 The background absorption was subtracted with a first-order polynomial over the pre-edge region, followed by normalization of the edge step. For the Cd K-edge XAS spectra, the energy scale was converted into k space, where k ) (8π2me/ h2)(E - E0), using a threshold energy of E0 ) 26710.8 eV for both solids. The EXAFS oscillation was then extracted using a sevensegment cubic spline. The EXAFS model functions, χ(k), were constructed by means of the FEFF 8.1 program,24,25 to obtain ab initio calculated amplitude feff(k)i, phase shift φij(k), and mean free path λ(k) functions (eq 1). The FEFF input file was generated by means of the ATOMS program,26 using structural information from the crystal structure of the reference compound Cd(SCH2CH2NH2)2, with both short and long Cd-S, Cd-N(/O), and Cd-Cd distances.16 This compound was suitable for creating different models, making use of the fact that the scattering parameters for the nitrogen and oxygen atoms are quite similar.

χ(k) )

∑ i

Ni·S02(k) k·Ri2

|feff(k)| i

exp(-2k2σi2) exp[-2Ri /λ(k)] sin[2kRi + φij(k)] (1) The structural parameters for each significant contribution to the χ(k) model function were refined by least-squares methods, fitting the k3-weighted χ(k) function to the experimental unfiltered EXAFS oscillations over the k ranges 2.4-16.0 Å-1, 2.4-12.0 Å-1, and 3.4-12.0Å-1,allowingthebonddistance(R),disorder(Debye-Waller) parameter (σ2), amplitude reduction factor (S02), and ∆E0 (correlated parameter for all scattering paths) to float, while the coordination number (N) was fixed. The fitting results for 2 are shown in Figure 3 and Figures S-4 and S-5 (Supporting Information) and Table 2 and Table S-3 (Supporting Information). In models I and VIIIa (Table 2), the estimated error limits for the refined Cd-S and Cd-O bond distances are (0.02 Å and (0.04 Å and for the corresponding disorder parameters are (0.001 and (0.003-0.005 Å2, respectively, including effects of systematic deviations.

Results Vibrational Spectroscopy. The vibrational bands attributed to the cadmium(II) complexes of 1 and 2 are almost identical (Figure 1 and Table S-1, Supporting Information). For 2, the Raman bands at 932 cm-1 (symmetric ClO4(23) Ressler, T. J. Synchrotron Radiat. 1998, 5, 118–122. (24) Zabinsky, S. I.; Rehr, J. J.; Ankudinov, A.; Albers, R. C.; Eller, M. J. Phys. ReV. B 1995, 52, 2995–3009. (25) Ankudinov, A. L.; Rehr, J. J. Phys. ReV. B 1997, 56, R1712-R1716. (26) Ravel, B. J. Synchrotron Radiat. 2001, 8, 314-316.

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stretching), 627 cm (antisymmetric ClO4 bending), and 460 cm-1 (symmetric ClO4- bending), as well as the IR absorption bands at 1117 and 627 cm-1, belong to the ClO4ion (Figure 1 and Table S-2, Supporting Information).27 To accommodate ClO4- in 2, a positively charged counterion is required, as the Cd(HCys)2 complex itself is neutral. Subtractions of the IR and Raman spectra of 1 from 2 (see Table S-2, Supporting Information) reveal some remaining spectral features besides those of the perchlorate anion. A strong IR band at 3210 cm-1 and minor residues at 1581 cm-1 and 1040 cm-1 can be assigned to the ν1, ν4, and ν2 fundamental modes of the H3O+ ion, respectively. For solid hydronium perchlorate, the Raman bands at 3285, 1571, and 1175 cm-1, assigned as the ν1, ν4, and ν2 modes of the pyramidal H3O+ ion,28 are in fairly good agreement with our observations. Hence, the main differences between the spectra of 1 and 2 are due to the presence of perchlorate and H3O+ ions in 2, for which the chemical composition {Cd(HCys)2 · H2O}2 · H3O+ClO4-, with one protonated water molecule, is proposed. In the difference Raman spectrum between 2 and 1, a very weak Raman band at 1736 cm-1 indicates hydrogen bonding between carboxylate and H3O+ ions in 2 (Table S-2, Supporting Information).29 The cysteine molecule has three potential coordination sites: the thiol (-SH), carboxylate, and amine groups. The cysteine S-H stretching band at 2554 cm-1 disappeared in the spectra of 1 and 2 (Table S-1 and Figure S-1, Supporting Information), indicating that all thiol groups are deprotonated and coordinated as thiolates to the cadmium(II) ion. The downshift in the Raman C-S stretching band from 694 cm-1 to 681-683 cm-1 (Figure 1) indicates that the coordination weakens the C-S bond.30 The strong bands at 1620-1624 cm-1 and 1490-1491 cm-1 in the IR spectra of 1 and 2, assigned as antisymmetrical and symmetrical NH3+ deformation modes, respectively,31 show that the amine group is protonated (-NH3+) and, therefore, not binding to the cadmium(II) ion (Table S-1, Supporting Information). The asymmetric and symmetric COO- stretching modes of cysteine at 1587 and 1394 cm-1 29 shift slightly in the IR spectra of 1 and 2 to νa (COO-) ) 1574-1575 cm-1 and νs (COO-) ) 1398-1400 cm-1. 13 C, 113Cd NMR Spectroscopy. 13C NMR spectra of solid cysteine and the compounds 1 and 2 are shown in Figure 2. The three sharp peaks in the spectrum of cysteine at δ(13C) (27) Nebgen, J. W.; McElroy, A. D.; Klodowski, H. F. Inorg. Chem. 1965, 4, 1796–1799. (28) Taylor, R. C.; Vidale, G. L. J. Am. Chem. Soc. 1956, 78, 5999–6002. (29) Sze, Y. K.; Davis, A. R.; Neville, G. A. Inorg. Chem. 1975, 14, 1969– 1974. (30) Susi, H.; Byler, D. M.; Gerasimowics, W. V. J. Mol. Struct. 1983, 102, 63–79. (31) Neville, G. A.; Drakenberg, T. Can. J. Chem. 1974, 52, 616–622. (32) Pickering, I. J.; Prince, R. C.; George, G. N.; Rauser, W. E.; Wickramasinghe, W. A.; Watson, A. A.; Dameron, C. T.; Dance, I. G.; Fairlie, D. P.; Salt, D. E. Biochim. Biophys. Acta 1999, 1429, 351– 364. (33) Isaure, M-P.; Fayard, B.; Sarret, G.; Pairis, S.; Bourguignon, J. Spectrochim. Acta, Part B 2006, 61, 1242–1252. ¨ z, G.; Pountney, D. L.; Armitage, I. M. Biochem. Cell Biol. 1998, (34) O 76, 223–234. (35) Bobsein, B. R.; Myers, R. J. J. Am. Chem. Soc. 1980, 102, 2454– 2455.

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Figure 2. (Left) 13C NMR spectra of the Cd(HCys)2 · H2O (1) and {Cd(HCys)2 · H2O}2 · H3O+ClO4- (2) amorphous solids compared with the spectrum of crystalline cysteine. The signals marked with * are spinning sidebands, and those with “c” are due to the cotton used to pack the sample (see Figure S-2, Supporting Information). (Right) 113Cd NMR spectra of 1 and 2. Table 1. 113Cd Chemical Shifts Reported for Cadmium(II)-Thiolates with CdSxOy Coordination Sitesa coordination

chemical shift (δ, ppm)

CdS4 (mononuclear) CdS4 (multinuclear) CdS3 (peptides) CdS3 (s) CdS3O (peptides) CdS3O2 (s) CdS3O3 (s) CdS4O (s) CdS4O2 (s) CdS2O4 (s) CdS2O2 a s ) solid; for solids, δiso and DMF.

reference

650, 680, 704-751 35, 57-60 600-707 34, 53, 61 572, 684-690 62-65 66, 67 590 (577b), 613, 668 560-645 62-64, 68 393, 400 69 391-409 39 481-507 39 245 this work 98, 123 14, this work 76, 144, 191 70, 71 ) isotropic shifts are reported; b In CDCl3

) 27.8, 55.7, and 173.2 ppm are ascribed to its Cβ(-S), CR, and carboxylate groups (COO-), respectively.18 The spectra of the cadmium(II) cysteinate complexes 1 and 2 appear very similar; the three peaks are broader than those of solid cysteine and are shifted downfield by 0.4, 0.8, and 0.7 ppm, respectively. These changes are associated with either the interaction of the cysteine functional groups with cadmium(II) ions (-S-, -COO-) or hydrogen bonding with the hydrating water molecules (-COO-, NH3+) or H3O+ ions (-COO-). The 113Cd NMR spectra of the amorphous solid compounds 1 and 2 are also similar, showing a single broad peak with its maximum in the 640-664 ppm region. The broadness of these peaks is attributed to the amorphous nature of the compounds: a range of isotropic shift (δiso) values are obtained for each sample. This is consistent with the broad 13 C peaks obtained (Figure 2). The chemical shifts (isotropic shifts for solids; δiso) for several types of CdSxOy coordination sites that have been reported are shown in Table 1. Given the broad peaks obtained for 1 and 2, the exact values for (36) Engeseth, H. R.; McMillin, D. R.; Otvos, J. D. J. Biol. Chem. 1984, 259, 4822–4826. (37) Meijers, R.; Morris, R. J.; Adolph, H. W.; Merli, A.; Lamzin, V. S.; Cedergren-Zeppezauer, E. S. J. Biol. Chem. 2001, 276, 9316–9321. (38) Bobsein, B. R.; Myers, R. J. J. Biol. Chem. 1981, 256, 5313–5316. (39) Jalilehvand, F.; Leung, B. O.; Mah, V. Inorg. Chem. Submitted.

Cadmium(II) Cysteine Complexes Table 2. Alternative Models Used for EXAFS Curve-Fitting of {Cd(HCys)2 · H2O}2 · H3O+ClO4- (2) in the k Range 2.4-16.1 Å-1 (see Figure S-4, Supporting Information)a Cd-S model

coordination model

N

R (Å)

Cd-O σ2 (Å2)

N

R (Å)

σ2 (Å2)

S02 b

∆E0

Rc

4f 2.515 0.0088 0.85 -1.6 24.0 3f 2.515 0.0088 1.13 -1.6 24.0 1f 2.403 0.0082 0.92 -3.4 22.1 3f 2.523 0.0066 IV CdS2S′2 2f 2.453 0.0105 0.91 -3.6 22.2 2f 2.531 0.0058 V CdS2S′ 1f 2.423 0.0094 1.22 -3.5 22.1 2f 2.526 0.0064 VI CdS4O 4f 2.523 0.0072 1f 2.257 0.0066 0.68 -0.7 21.6 VII CdS4O2 4f 2.528 0.0070 2f 2.300 0.0106 0.62 0.6 21.5 VIIIa CdS3O 3f 2.525 0.0070 1f 2.273 0.0079 0.87 -0.2 21.5 VIIIb 3f 2.528 0.0074 2.1 2.350 0.0166 0.85 f 1.5 21.4 IX CdS3O2 3f 2.525 0.0087 2f 2.434 0.0298 1.10 1.0 21.0 X CdS3O3 3f 2.527 0.0089 3f 2.466 0.0371 1.14 1.4 19.9 XI CdS2O2 2f 2.527 0.0089 2f 2.466 0.0371 1.71 1.4 19.9 XII CdS2O4 2f 2.542 0.0048 4f 2.347 0.0100 0.70 4.0 26.2 a N ) coordination number; f ) fixed; b For model VIII b, the S02 was fixed at 0.85, a value obtained from EXAFS fitting of standard complexes.39 c The residual (%) from the least-squares curve fitting is defined as {[∑Ni)1|yexp(i) - ytheo(i)|]/[∑Ni)1|yexp(i)|]} × 100, where yexp and ytheo are experimental and theoretical data points, respectively. I II III

CdS4 CdS3 CdS3S′

δiso are uncertain, but the isotropic values fall in the 500-700 ppm range, which is a region characteristic for CdS4, CdS3, CdS3O, and CdS4O coordination geometries (see Table 1). The vibrational spectra showed protonated amine groups in both 1 and 2, and therefore, CdSxNy coordination is not possible in these compounds. Cd K-Edge EXAFS Spectroscopy. To further examine 1 and 2, their Cd K-edge EXAFS spectra were measured. The complete overlap of the two EXAFS oscillations (Figure S-3, Supporting Information) indicated the same local structure around the cadmium(II) ions in both solids. Because of the similarity of the spectra, only the results for compound 2 are discussed. Different data k ranges (2.4-16.1 Å-1, 2.4-12.0 Å-1, and 3.5-12.0 Å-1) and several coordination models were used for the least-squares model fitting (Table 2, Tables S-3a and S-3b and Figures S-4 and S-5, Supporting Information). For all models in different data k ranges, the average Cd-S bond distance of 2.52 ( 0.02 Å consistently emerged (Table 2 and Tables S-3a and S-3b, Supporting Information). The average Cd-O distance for CdS3O, CdS4O, and CdS4O2 models (VI-VIIIa) varied between 2.22 and 2.30 Å using different k ranges in the refinements. Within each data k range, the differences in the residuals were not conclusive. Figure 3 shows the results for the CdS3O model (VIIIa in Table 2), including the separate contributions of the Cd-S and Cd-O scattering paths. A small peak at ∼3.5 Å (not corrected for the phase shift) was observed in the Fourier transform (FT) of the EXAFS spectra of 1 and 2 (Figure 3). Attempts to include a Cd-Cd scattering path in models I and VIIIa were unsuccessful in reproducing the feature properly, showing a contribution of less than 2% for this path. For oligomeric complexes with a double thiolate bridge such as bis(cysteaminate)cadmium(II) [Cd(SCH2CH2NH2)2], the well-defined Cd · · · Cd distance at 3.64 Å gives rise to a distinct FT peak (Figure S-6, Supporting Information). However, for the cadmium adamantane cage structure, (Et3NH)4[S4Cd10(SPh)16], with single thiolate bridges between the cadmium(II) ions, no peak could

Figure 3. Least-squares curve-fitting of the k3-weighted EXAFS spectrum of 2 for a CdS3O model (VIIIa, Table 2) and the corresponding Fourier transforms, with separate Cd-S and Cd-O contributions.

be discerned for Cd · · · Cd interactions between 3 and 4 Å (Figure S-6a, Supporting Information). Cd L3-Edge XANES Spectroscopy. Cd L3-edge XANES spectra have been proposed to be sensitive to the local structure and type of the coordinating atoms in cadmium(II) complexes.32,33 Therefore, we prepared and measured the Cd L3-edge XANES spectra of several crystalline CdSx(O/ N)y complexes with different coordination geometries (see the Experimental Section and Figure S-7, Supporting Information), for comparisons with the indistinguishable spectra of 1 and 2 (Figure 4). The distinct pre-edge peak at 3539.1 eV in the spectrum of Cd(ClO4)2 · 6H2O (CdO6 model) gradually blends into the absorption edge of the spectra of the standard compounds used as CdS2O4 (I and II), CdS3O3, CdS6, CdS3O, and CdS2N2 models and finally disappears in the CdS3N2 and CdS4 spectra. This pre-edge feature is due to a transition from Cd 2p3/2 to unoccupied molecular orbitals with mainly s and d character. The Cd L3-edge XANES spectra of 1 and 2 display two shoulders at 3539.1 and 3541.3 eV, which become more distinct in the second derivative (see Figure 4). The shapes of the absorption edge features for 1 and 2 and their corresponding second derivatives appear intermediate to Inorganic Chemistry, Vol. 48, No. 9, 2009

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Figure 4. Cd L3-edge XANES spectra and the corresponding smoothed second derivatives of 1 and 2, compared with those of crystalline CdSx(N/O)y complexes used as coordination models (see Figure S-7, Supporting Information). Dashed lines are at 3539.1 and 3541.3 eV.

those of the CdS3O and CdS3N2 standards. Since the scattering parameters for the nitrogen and oxygen atoms are quite similar, the CdS3N2 model may represent the CdS3O2 configuration as well. Discussion The compositions of the two solid compounds obtained through the reactions of cysteine with Cd(CH3COO)2 · 2H2O or Cd(ClO4)2 · 6H2O were characterized as Cd(HCys)2 · H2O (1) (precipitated at pH ) 2.7) and {Cd(HCys)2 · H2O}2 · H3O+ClO4- (2) (precipitated at pH ) 1.6), respectively, based on C, N, and H elemental analyses and thermal gravimetric and IR/Raman measurements. The similarity of the Cd K-edge EXAFS, Cd L3-edge XANES, and 13C NMR spectra of 1 and 2 and the comparable 113Cd NMR spectra showed closely related structures for the cadmium(II) cysteine complexes in 1 and 2. The different composition of the cadmium(II)-cysteinate compound 2 seems to be connected to the low pH value at which it precipitated. A similar inclusion of acid in a crystal structure has been reported for the Hg(II) cysteinate complex, Hg(HCys)2 · HCl · 1/2H2O, prepared by reacting cysteine and HgCl2.31 The broad signals obtained from the solid-state 113Cd NMR spectra of the amorphous compounds 1 and 2 (span in the 500-700 ppm range) with maxima at ∼650 ppm (Figure 2) appear in a region characteristic for CdS4 (especially in single-bridged multinuclear cadmium(II)-tetrathiolates, as in Cd(II) loaded metallothionein 600-680 ppm),34 CdS3, CdS3O, and CdS4O coordination geometries (see Table 1). For a NMR spectrum of a powder sample, the isotropic chemical shift is not necessarily the point of maximum intensity in the spectrum: the line shape one obtains depends on the magnitudes of the principal components (δ11, δ22, and δ33) of the chemical shift tensor that describes the anisotropic

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magnetic shielding; this in turn depends on molecular properties and symmetry. This is clear from the 113Cd NMR spectrum of the Cd(tu)2(NO3)2 complex (CdS2O4), where the peak maximum is at ∼280 ppm, while δiso ) 123 ppm (Figure S-8, Supporting Information). The 113Cd NMR spectra of 1 and 2 look very different from that previously reported for the dehydrated Cd(HCys)2 solid compound, which showed narrow peaks within the ∼180-620 ppm region with a δiso value of 402 ppm, and was attributed to a chain structure with CdS4O2 units (Scheme 1, left). Such narrow bands are not normally consistent with an amorphous solid, and Barrie et al. interpreted the spectrum as indicating a “high degree of local order at the cadmium site”.9 Also, the assignment to a polymeric chain with CdS4O2 units seems doubtful for a dehydrated Cd(HCys)2 compound. The only similar polymeric structure that we could find in the crystal structure database was that of crystalline [Cd(tu)2(HCOO)2] (tu ) thiourea), where thiourea acts as a bridging ligand between the cadmium(II) ions, with two sets of Cd-S distances of 2.709 and 2.740 Å and a Cd-O distance of 2.277 Å to the formate ions.17 We measured the solid-state 113Cd NMR spectrum of this complex (Figure S-9, Supporting Information) and found the isotropic chemical shift δiso ) 245 ppm. To the best of our knowledge, this is the first 113Cd chemical shift reported for a complex with CdS4O2 geometry, which is not consistent with the proposed assignment of the δiso value of 402 ppm for the dehydrated Cd(HCys)2 solid compound to CdS4O2 coordination geometry. The distances to the different types of atoms coordinated to the cadmium(II) ion are essential when discussing the structures of 1 and 2. EXAFS spectroscopy provides such information for the local structure around the central absorbing atom. Since the protonated amine groups of the cysteine

Cadmium(II) Cysteine Complexes Table 3. Survey of a Series of Cadmium(II) Complexes with S-Donor Ligands (distances are in ångstroms; from CSD version 5.29, Nov 2007; see Tables S-5 and S-6 in Supporting Information for details)

a

coordination

No. of crystal structures

Cd-S distance range

Cd-S average distance

CdS3 CdS4 (mononuclear) CdS3O CdS3N CdS2N2 CdS2O2 CdS2O3 CdS3O2

4 5 3 1 10 2 2 1

2.376-2.582 2.515-2.554 2.493-2.573 2.405-2.588 2.417-2.560 2.634-2.767 2.460-2.529 2.508-2.870

CdS3N2

5

2.454-3.109

CdS3O3 CdS2O4 CdS4O2 CdS4N2

2 7 3 3

2.533-2.602 2.497-2.647 2.646-2.740 2.543-3.038

2.446 2.531 2.527 2.522 2.473 2.701 2.494 2.522 (2.637)a 2.551 (2.622)a 2.560 2.557 2.683 2.664 (2.716)a

Cd-(O/N) distance range

Cd-(O/N) average distance

2.108-2.308 2.207 2.226-2.342 2.102-2.121 2.258-2.610 2.221-2.374

2.240 2.207 2.288 2.112 2.392 2.297

2.308-2.477

2.386

2.291-2.752 2.266-2.794 2.277-2.470 2.331-2.479

2.550 2.434 2.385 2.394

Average obtained by including longer, bridging Cd-S distances.

ligands in 1 and 2 are not able to coordinate to the cadmium(II) ion, we restricted our evaluation to CdSxOy coordination models. Several structural models, including CdS4, CdS3, CdS3O, CdS4O, CdS3O2, CdS3O3, CdS2O2, CdS4O2, and CdS2O4, were used for least-squares curvefitting to the k3-weighted EXAFS spectra of 1 and 2 in different k ranges: k ) 2.4-16.1, 2.4-12.0, and 3.5-12.0 Å-1. Almost all models tested were able to provide good fits in both k and r space (Figures S-4 and S-5, Supporting Information) because of the dominating backscattering from the sulfur atoms. The average Cd-S bond distance obtained from all model fittings was consistently 2.52 ( 0.02 Å (Table 2 and Tables S-3a and S-3b, Supporting Information). The minor Cd-O contribution was more difficult to analyze because of the dominating backscattering from the sulfur atoms. For the CdS3O, CdS4O, and CdS4O2 models (VI-VIIIa), the average Cd-O distance and corresponding disorder parameter varied between 2.22 and 2.30 Å and 0.005 and 0.013 Å2, when fitting the EXAFS oscillation at different k ranges. The CdS4O (VI) and CdS4O2 (VII) models resulted in unreasonably small amplitude reduction factors (S02) of 0.6-0.7. For the CdS3O2, CdS3O3, CdS2O2, and CdS2O4 models (IX-XII), the Cd-O distance fluctuated drastically between 2.28 and 2.48 Å by changing the fitting k range and was accompanied by unstable disorder parameters (σ2 from 0.008 to 0.05 Å2) and S02 values. Those extreme changes in Cd-O distances, σ2(Cd-O), and S02 values indicated that the IX-XII models did not properly represent the Cd(II) sites in 1 and 2. The main conclusion derived from the above EXAFS data analyses is that the mean Cd-S distance is 2.52 ( 0.02 Å for 1 and 2. To identify the dominating type of coordination environment, the mean Cd-S bond distance obtained from the EXAFS spectra was used for comparisons with those in structurally known cadmium(II) complexes with S-donor ligands, aided by the broad signal observed in their solidstate 113Cd NMR spectra in the 500-700 ppm region, with peak maxima around 650 ppm. Such comparisons are more likely to provide reliable information on the coordination geometry of the cadmium(II) ion in 1 and 2 than the coordination numbers obtained from EXAFS, which have

low accuracy because of the correlation with other parameters affecting the amplitude of an EXAFS oscillation, that is, the amplitude reduction factor (S02) and the Debye-Waller (disorder) parameter (σ2). Table 3 provides ranges and average values of the Cd-S and Cd-(O/N) bond distances in a series of crystalline cadmium(II) complexes with S-donor ligands (structural details are provided in Tables S-5 and S-6 in the Supporting Information). As shown in Table S-6, the nature and denticity of the ligands strongly influence the distribution of metal-ligand bond distances. Some coordination figures have only been found with S ligands other than thiolates, such as thiourea. For some crystal structures, for example, dinuclear complexes with CdS3O2, CdS3N2, or CdS4N2 coordination, shown in Table 3 and Table S-5, the distribution of Cd-S bond distances is wide since the S-donor ligands act as both terminal and bridging groups. In such cases, two mean values of the Cd-S distances have been provided, with and without accounting for the contribution from the long Cd-S distances. When comparing the average of a range of distances obtained from EXAFS data with those determined by crystallography, care should be taken, as shorter distances generally have a higher contribution to the EXAFS oscillation (due to the 1/R factor in eq 1, and often lower σ2 values). Possible CdSxOy Coordination Sites for the Amorphous Solids 1 and 2. CdS2O2 Coordination. The 113Cd NMR chemical shifts reported for CdS2O2 coordination in Table 2 are obtained for S-donor ligands other than thiolates and may not be fully comparable with those of 1 and 2, which span 500-700 ppm. A more appropriate estimate of the expected 113Cd chemical shift for CdS2O2 coordination with thiolates can be obtained by considering the 113Cd chemical shifts found for cadmiumsubstituted horse liver alcohol dehydrogenase (LADH), and its binary complex with nicotinamide adenine dinucleotide, LADH-NAD(H). In cadmium(II)-LADH, the Cd(II) ion occupies a CdS2NO coordination site, δ(113Cd) ) 483 ppm, where it is surrounded by two cysteine thiolates, one imidazole nitrogen, and one water molecule.35,36 For the cadmium(II)-substituted LADH-NAD(H), for which the 1-Å-resolution X-ray structure revealed a CdS2NO2 coordiInorganic Chemistry, Vol. 48, No. 9, 2009

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113

nation site, the Cd NMR resonance becomes shielded (442-443 ppm) relative to that of cadmium(II)-LADH (483 ppm).38 Nitrogen is more deshielding than oxygen, and removing the nitrogen ligand from a CdS2NO2 site would further shift the 113Cd signal upfield. Therefore, for a cadmium(II) thiolate complex with a stable CdS2O2 coordination environment, a 113Cd chemical shift of ∼400 ppm would be expected, which still is shielded relative to the broad 500-700 ppm signal of 1 and 2. Moreover, using this model for EXAFS curve-fitting of 1 and 2 resulted in very high values of S02 and disorder parameters for the Cd-O scattering path (XI, Table 2). Hence, CdS2O2 coordination is not likely for the cadmium(II) site in 1 and 2. CdS2O3 Coordination. For this type of coordination, the Cd chemical shift is expected to be more shielded than for CdS3O2 coordination (,400 ppm, Table 1), as an oxygen ligand atom is more shielding than sulfur. Therefore, such a coordination environment is unlikely in 1 and 2, even though the average Cd-S bond distance for CdS2O3 coordination, ∼2.49 Å (Table 3), is close to the mean Cd-S distance 2.52 ( 0.02 Å obtained for 1 and 2. 113

CdS2O4 Coordinations. Also, the mean Cd-S bond distance for this type of coordination (∼2.56 Å) is fairly close to that obtained for 1 and 2. EXAFS curve fitting using this model over the k range ) 2.4-16.1 Å-1 gave reasonable values for the refined parameters (XII, Table 2). However, reducing the fitting range to 2.4-12 Å-1 and 3.5-12 Å-1 resulted in very small (0.61), or very large, S02 values (1.55), respectively, indicating an inadequate model. The reported 113 Cd chemical shift values (∼100 ppm; Table 1) for CdS2O4 compounds (S-donor ligand ) thiourea, tetramethylthiourea) are considerably more shielded than the broad 113Cd NMR signals of 1 and 2 (∼450-780 ppm). Therefore, on the basis of the combined NMR and EXAFS results, CdS2O4 coordination is not expected in 1 and 2. CdS3 Coordination. The average Cd-S bond distance for cadmium(II) complexes with CdS3 coordination is ∼2.45 Å (Table 3), which is considerably shorter than the mean Cd-S distance of 2.52 ( 0.02 Å for 1 and 2. Therefore, this type of coordination mode is unlikely, even though the reported 113Cd chemical shift for CdS3 coordination falls within the broad 113Cd NMR signals of 1 and 2. CdS3O2 and CdS3O3 Coordination. The average Cd-S bond distances for CdS3O2 and CdS3O3 coordination, ∼2.52 and 2.56 Å, respectively, do not differ significantly from the mean Cd-S distance of 2.52 ( 0.02 Å for 1 and 2. Leastsquares curve-fitting of the EXAFS oscillations for 1 and 2 using these models at different k ranges showed very large disorder parameters (σ2) for the Cd-O scattering path, as well as highly fluctuating Cd-O bond distances and S02 values (IX and X in Table 2 and Table S-3a,b, Supporting Information). Fixing the coordination number NCd-S ) 3 and keeping S02 at 0.85, a value obtained from EXAFS fitting of standard complexes,39 resulted in refined NCd-O values between 0.8 and 2.1 (depending on the k-fitting range), RCd-O ) 2.24-2.35 Å, and σ2 ) 0.006-0.016 Å2 (model VIIIb, Table 2 and Table S-3a,b, Supporting Information). The

4226 Inorganic Chemistry, Vol. 48, No. 9, 2009

reported chemical shifts for these coordination geometries are about 400 ppm, which is rather shielded relative to the broad 113Cd NMR signals of 1 and 2. Since both NMR and EXAFS spectra provide average values for all possible cadmium(II) ion sites in the sample, a minor amount of such coordination geometries in 1 and 2 cannot be ruled out. CdS4O Coordination. The decanuclear cadmium complex of the compound [Cd10(SCH2CH2OH)16](ClO4)4 · 8H2O has CdS4O coordination sites with average Cd-S and Cd-O bond distances of 2.594 and 2.410 Å, respectively.40 The corresponding mean distances obtained when using this type of model (VI) for EXAFS fittingsCd-S, 2.52 ( 0.02 Å and Cd-O, 2.22-2.26 Å (Table 2 and Table S-3a,b, Supporting Information)sare considerably shorter. The reported 113Cd chemical shift of 481-507 ppm falls within the region where the 113Cd NMR signals for 1 and 2 appear (500-700 ppm). Therefore, a minor amount of cadmium(II) complexes with such coordination is possible in 1 and 2. CdS4O2 Coordination. The proposed structure by Barrie et al. for a dehydrated Cd(HCys)2 compound has such a coordination environment (Figure 1, left).9 The solid-state 113 Cd NMR isotropic shift of 245 ppm for the [Cd(tu)2(HCOO)2] complex with CdS4O2 coordination (Figure S-9, Supporting Information) shows considerable shielding relative to the 113Cd NMR signals of 1 and 2. Also, the average crystallographic Cd-S bond distance in CdS4O2 complexes (∼2.68 Å; Table 3) is much longer than the mean Cd-S distance of 2.52 ( 0.02 Å, obtained for 1 and 2. EXAFS curve-fitting of 1 and 2 using this model (VII) resulted in a fairly constant Cd-O distance (2.26-2.30 Å) but an unrealistic, small S02 value (0.6). Therefore, CdS4O2 coordination, with its high shielding and long mean crystallographic Cd-S distance, is unlikely for the cadmium(II) ions in 1 and 2. CdS4 and CdS3O Coordination. We have recently reported the Cd L3-edge spectra of a series of cadmium(II) cysteine solutions (pH 11.0), with an increasing excess of cysteine. The solutions with molar ratios CH2Cys/CCd(II) of 15-20 and 113Cd chemical shifts of 654-658 ppm were found to contain predominantly [Cd(Cys)4]6- (CdS4) complexes, with very similar features in the second derivative of their Cd L3-edge spectra to those of the standard CdS4 compound.39 However, the different relative sizes of the corresponding two main features for 1 and 2 (Figure 4) to some extent resemble those of the CdS3O standard model compound and indicate that CdS4 cannot be the only type of coordination occurring in 1 and 2. Attempts to describe the k3-weighted EXAFS spectra of 1 and 2 with a model including only Cd-S backscattering, that is, as for CdS4 coordination, showed a relatively poor fit at high k values (k > 10.5 Å-1) and r ∼ 1.5 Å, when fitting the data in the k range 2.4-12.0 Å-1 (model I, Figure S-5, Supporting Information), while including Cd-O in CdS3O coordination (VIIIa) improved the fitting residual. Both models resulted in average Cd-S distances of 2.52 ( (40) Lacelle, S.; Stevens, W. C.; Kurtz, D. M.; Richardson, J. W.; Jacobson, R. A. Inorg. Chem. 1984, 23, 930–935.

Cadmium(II) Cysteine Complexes Table 4. Assignment of Far-Infrared and Raman Bands (cm-1) of 1 and 2 (see Figure 5) Cd(HCys)2 · H2O (1) {Cd(HCys)2 · H2O}2 · H3O+ClO4- (2) IR

Raman

687 m, w 683 s 622 w 629 w

IR

531 vs 521 m 464 w, sh 467 m, sh

670 w a 625 vs 531 vs 464 w

420 vw 365 s

420 vvw 365 s

420 vvw 372 vw 275 s

(∼275) 255 s, sh

assignment

681 vs a 627 s 531 w, m a 463/458m 420 vvw ∼360 vvw 278 s

νCS, δCO2 ωCO2 ν4[ClO4]FCO2, δNCC δNCC ν2[ClO4]τNH3+/δNCC δCCS νsCdS terminal νaCdS terminalb νaCdS bridgingc νsCdS bridging νaCdS bridging δOCdO, δOCdS

(∼278) ∼250 m, b 239 w, sh

221 vs, b 150 m, sh

Raman

244 m, sh 228 vs, b 164 m, sh

a Overlapping with [ClO4]- bands. b Estimated values close to νsCdS Raman features. c An alternative assignment of these bands to the Cd-O stretching of weakly coordinated carboxyl oxygen.

0.02 Å, which is close to that of the crystalline cadmium(II) complexes with tetrahedral CdS4 (mononuclear) and CdS3O coordination (Cd-Save ) 2.53 Å) in Table 3. Among all CdSxOy models used for the EXAFS curve-fitting (VI-XII), only the CdS3O (model VIIIa) consistently yielded plausible sets of refined parameters over different fitting k ranges. The Cd-O distance obtained values in the range 2.24-2.27 Å, compatible with that observed for CdS3O crystalline compounds (Table 3), with a corresponding reasonable disorder parameter σ2 ) 0.007-0.01 Å2. While the amplitude reduction factor (S02 ) 0.8-0.9) became realistic for both models, the relatively high disorder parameter for the Cd-S scattering pathway in model I (σ2 ) 0.008(1) Å2) indicated a fairly large distribution of the Cd-S distances. An attempt to resolve the Cd-S distances by means of the CdS3S′ and CdS2S′2 models (III and IV, Table 2 and Table S-3a,b, Supporting Information) was not successful, leading to higher σ2 values for the shorter Cd-S distances. However, the Cd-S vibrational bands in the Raman and far-IR spectra can be used as a probe for terminal and bridging Cd-S bonds. A single Cd-S stretching Raman band has been observed at 269 cm-1 for a cadmium(II)cysteine complex41 and at 282 cm-1 for a cadmium(II)-bound zinc-finger peptide.42 The Raman bands at 275 and 278 cm-1 of 1 and 2, respectively, can similarly be attributed to symmetric stretching of the Cd-S terminal bonds (Table 4, Figure S-10, Supporting Information). The asymmetric terminal Cd-S stretching band is expected to be very close to the Raman band at 275 cm-1 due to an absence of vibrational coupling between the terminal Cd-S bonds, but it is difficult to locate its position in the very broad and overlapping infrared features between 300 and 150 cm-1 (Figure 5). The strongest IR bands for 1 and 2 in the far-IR region, at 221 and 228 cm-1, respectively, are the best candidates for the bridging Cd-S antisymmetric stretching. Close to the positions of these far-IR bands, the Raman spectra exhibit shoulders at 239 and 244 cm-1, which can (41) Faget, O. G.; Felcman, J.; Giannerini, T.; Te´llez, S. C. A. Spectochim. Acta, Part A 2005, 61, 2121–2129. (42) Vargek, M.; Zhao, X.; Lai, Z.; McLendon, G. L.; Spiro, T. G. Inorg. Chem. 1999, 38, 1372–1373.

Figure 5. Far-infrared spectra {Cd(HCys)2 · H2O}2 · H3O+ClO4- (2).

of

Cd(HCys)2 · H2O

(1)

and

be assigned as symmetric stretches of bridging Cd-S (Table 4, Figure S-10). Beside those, the shoulders in the IR spectra for 1 and 2 at 255 and 250 cm-1, respectively, can also be assigned to the antisymmetric vibrational modes of the bridging Cd-S bonds. Therefore, we propose an assignment where the higher-frequency bands at ∼275 cm-1 belong to the stretchings of the terminal Cd-S bonds and the lowerfrequency bands between 255 and 221 cm-1 to the stretching modes of the weaker bridging Cd-S bonds (Table 4). A closer look at low-frequency bands (